The principal components of a synchronous motor are the stator and the rotor. The stator of synchronous motor and stator of induction motor are similar in construction. With the wound-rotor synchronous doubly fed electric machine as the exception, the stator frame contains wrapper plate. Circumferential ribs and keybars are attached to the wrapper plate. To carry the weight of the machine, frame mounts and footings are required. When the field winding is excited by DC excitation, brushes and slip rings are required to connect to the excitation supply. The field winding can also be excited by a brushless exciter. Cylindrical, round rotors, (also known as non salient pole rotor) are used for up to six poles. In some machines or when a large number of poles are needed, a salient pole rotor is used. The construction of synchronous motor is similar to that of a synchronous alternator.